(1) FIELD OF THE INVENTION
This invention relates to a heat-sensitive and heat-transferable recording sheet.
(2) DESCRIPTION OF PRIOR ARTS
In recent years, a heat-transferable recording method having advantages such as storability, indelibility and solvent resistance of recorded image has been practically used in parallel with a heat-sensitive recording method employing a heat recording apparatus such as a thermal printer, a thermal facsimile or the like. The heat-transferable recording method uses a heat-transferable recording sheet comprising a subtrate and a heat-meltable ink layer provided thereon. The ink layer side of the heat-transferable recording sheet and a plain paper are superimposed, and the ink of the recording sheet is transferred onto the plain paper by using a heat supplied from a thermal head of a thermal facsimile or the like, whereby recording is conducted. Heat-transferable recording in a single color (e.g. black color) is already in practical use.
There is also a heat-sensitive and heat-transferable recording sheet enabling multiple recording which comprises a substrate, a heat-meltable ink layer (heat-transferable layer) provided on one side of the substrate and a heat-sensitive, color-forming layer provided on the other side of the substrate. In this recording sheet, the heat-transferable layer has a color because the heat-meltable ink used in the layer is a color dye or pigment, and the color is seen through even at the heat-sensitive, color-forming layer side. This is undesirable from the appearance standpoint and makes an image developed on the heat-sensitive, color-forming layer difficult to distinguish clearly. Hence, various improvements have been made for the heat-sensitive and heat-transferable recording sheet.
For example, Japanese patent application Kokai (Laid-open) No. 78793/1983 discloses a thermal recording sheet for multiple recording comprising a substrate, a heat-sensitive, color-forming layer provided on one side of the substrate and a heat-meltable color ink layer provided on the other side of the substrate. However, in order for the color of the heat-meltable ink not to be seen through the substrate at the heat-sensitive, color-forming layer side, the substrate is subjected in advance to vapor deposition of metal.
Further, Japanese patent application Kokai (Laid-open) No. 126482/1980 discloses a heat-sensitive recording sheet for multiple recording comprising a substrate, a heat-sensitive, color-forming layer provided on one side of the substrate and a heat-sensitive, meltable, color-forming, transferable layer containing a combination of a colorless leuco dye and a phenolic compound, provided on the other side of the substrate.
Further, Japanese patent application Kokai (Laid-open) No. 117993/1982 discloses a thermal recording medium comprising a base paper, a heat-sensitive ink layer provided on one side of the base paper and a heat-sensitive, color-forming, transferable layer provided on the other side of the base paper. The heat-sensitive, color-forming, transferable layer is composed of Crystal Violet Lactone, bisphenol A and a wax (e.g. paraffin wax, rice wax).
The recording sheets disclosed in the above laid-open patents each have drawbacks as follows.
In the recording sheet of Japanese patent application Kokai (Laid-open) No. 78793/1983, the substrate need be subjected to metal deposition and it requires one additonal production step and incurs cost increase. Further, since a heat-meltable color ink is used in the heat-transferable layer, there occurs, when the recording sheet and an image-receiving sheet are superimposed for storage, the transfer of the color ink onto the image-receiving sheet and the resulting staining of the image-receiving sheet.
In the recording sheet of Japanese patent application Kokai (Laid-open) No. 126482/1980, a phenolic compound is used for reaction with a leuco dye. The phenolic compounds disclosed therein for said purpose are generally very reactive and easily develop a color in mere mixing with a leuco dye. This causes background fogging of coated surface in the step of coating and drying of a combination of the phenolic compound and the leuco dye.
In the recording sheet of Japanese patent application Kokai (Laid-open) No. 117993/1982, the heat-sensitive, color-forming, transferable layer is composed of Crystal Violet Lactone, bisphenol A and a wax. In coating of these components on a base paper and drying of the resulting layer, there easily occurs background fogging due to the hot air stream used for the drying, making it difficult to obtain a desired white background. Further, since transfer to a plain paper is conducted by using a heat supplied by a thermal head through the base paper to allow the heat-sensitive, color-forming, transferable layer to develop a color, it is necessary for satisfactory transfer to use a large heat amount or to allow the heat-sensitive, color-forming, transferable layer to have a high sensitivity. However, the high sensitivity causes background fogging in the drying step of said layer after coating.
Hence, the first object of the present invention resides in providing a heat-sensitive and heat-transferable recording sheet having a white background, free from the above drawbacks and having no background fogging.
Heat-sensitive and heat-transferable recording sheets are in use in a large amount in recent years because recording can be made simultaneously on said recording sheet and a plain paper by one time recording with a thermal head.
These recording sheets find a major application in chits used in heat recording apparatuses of easily portable type. The recording sheets, however, have a color and this is undesirable from the appearance standpoint and causes pressure fogging or friction smudge. Therefore, a heat-sensitive and heat-transferable recording sheet of a type using a colorless ink has come to be preferably used. There is yet desired a heat-sensitive and heat-transferable recording sheet not only capable of conducting recording with a thermal head but also having such a pressure sensitivity as enables signature by a writing pressure which is necessary when the sheet is used as a chit and the like.
The present inventors had previously invented a heat-sensitive and heat-transferable recording sheet having a pressure sensitivity and had filed a patent application for the recording sheet in Japanese patent application No. 25329/1984. In this record sheet, the whole portion of the colorless, heat-transferable layer has a sensitivity. Therefore, the portions of the heat-transferable layer other than those truly requiring a pressure sensitivity cause friction smudge and moreover such a heat-transferable layer incurs a higher cost. Hence, an improvement is desired.
Therefore, the second object of the present invention resides in providing a heat-sensitive and heat-transferable recording sheet having a pressure sensitivity only at the selected, necessary places which is free from friction smudge and yet is inexpensive and wherein the above mentioned drawbacks caused by a heat-transferable layer whose whole portion has a pressure sensitivity have been improved.